The present invention relates to a toroidal deflection yoke core in the deflection unit of a cathode ray tube, also referred to as yoke ring.
One of the parts of a cathode ray tube (CRT) is the deflection unit (DU), holding the deflection coils which generate a magnetic field to deflect electrons from the cathodes to the appropriate points on the screen. The ring-shaped deflection yoke, normally made of ferrite, surrounds the deflection coils in order to keep the magnetic field inside the deflection volume. For a perfectly symmetrical yoke, the magnetic field is amplified, as multipoles generated by the coils are reflected by the yoke.
The position on the screen and the landing angle at which the electrons arrive on the screen vary approximately linearly with the current in the coils (Gaussian approximation). In reality, however, this is not a perfect approximation and generates significant errors in situations where large currents are needed.
Such errors are a serious problem when designing slim tubes, where the deflection anglesxe2x80x94and consequently the coil currentsxe2x80x94need to be increased.
Several techniques are used to reduce these errors, such as positioning the coil wires optimally, providing magnets to the deflection core, etc. However, none of these techniques can provide a completely satisfying result, especially for high ( greater than 120 degs) deflection angles.
It is an object of the present invention to overcome the above problem, and provide a deflection yoke for a CRT which improves front-of-screen (FOS) performance.
According to the invention, this and other objects are achieved with a yoke ring having an inner and an outer contour, characterized in that said inner contour is periodically deformed in the radial direction, the contour having at least two local minima and maxima.
The invention is based on understanding the behavior of the magnetic field. With a normal deflection yoke, multipoles generated by the coils are reflected in the yoke, which acts like a mirror, amplifying the field. By influencing these multipole reflections, a yoke ring according to the present invention shows improved performance.
In a standard, completely symmetrical and circular yoke ring, each multipole is reflected as the same multipole but with a reduced amplitude. By periodically deforming the inner contour of the yoke ring, e.g. the boundary against which the multipoles are reflected, an n-th order multipole will not only scatter as an n-th order multipole, but several multipoles of higher and lower order will be generated. It is the influence of these additional multipoles that increases the FOS performance.
In first-order perturbation theory, the interaction between the field generated by the coils, represented by a scalar potential "PHgr", and the perturbation xcex5(xcex8)(=deformed radiusxe2x88x92undeformed radius at the point with angular position xcex8) of a circular yoke boundary can be described as follows:             ϵ      ⁡              (        θ        )              ⁢                  ∂        Φ                    ∂        n              ,
where n stands for normal. This term describes a first order correction to the boundary potential on the circle. It can be shown that by deforming the radius of a circular yoke with the modulation cos(mxcex8), an n-th order multipole scatters not only as an n-th order multipole, but additionally an n+m-th and an |nxe2x88x92m|-th order multipole are generated.
Tests proved that the periodic deformations according to the invention have a positive effect on the FOS performance, reducing astigmatism, coma and raster errors. The astigmatism error refers to the relative position of the blue and red beams with respect to one another. The coma error refers to the difference between the arithmetic average of the blue and red beams and the green beam. These errors are associated with different Fourier components of the magnetic field, where the dipole is associated with the raster error, the quadrupole with the astigmatism error, and the six-pole with the coma error.
The periodic variations in the inner contour are formed around an original diameter, which is a constant in the most common, circular case. However, non-circular yoke rings also exist, in which case the periodic deformations are formed around this non-circular contour. Note that, in this case, the terms local minima and maxima are reserved for the periodic variations. The non-circular basic shape, e.g. an elliptic shape, is thus not considered to have local minima and maxima in the sense of the current invention.
In mathematical terms, the deformations can be regarded as transformations (in polar coordinates) of each point on the yoke boundary:
(r, xcex8, z)xe2x86x92(r+f(xcex8), xcex8, z), 
where f(xcex8) is a periodic function.
A further advantage of the inventive deformation of the yoke is that it can be used together with all existing techniques for improving FOS performance.
According to a preferred embodiment of the invention, the outer contour is also periodically deformed in a similar way. Although having less impact, these deformations further improve FOS performance. In the latter case, the periodic deformations of the inner and outer contours may be equal, resulting in a constant distance between the boundaries. Tests indicate that this has a positive effect on the improvements of the FOS performance.
The amplitude of the periodic function, i.e. the difference between local minima and maxima may be dependent upon the z value, which is defined as the position along the central axis of the yoke ring. It is also possible to let the amplitude be zero for a substantial part of the yoke axis, resulting in periodic deformations only along a portion of the axial length. A minimum of 10% of the axial length should, however, be deformed in order to achieve the desired effect.
It has been found that correction on the color errors (astigmatism and coma errors) can be obtained mainly by deforming the yoke on the neck side. This is due to the fact that here the coils and the yoke ring lie closer to the electron trajectories and that the influence of the six-pole Fourier component of the field on the coma error is greatest on the neck side. Similarly, raster errors can be corrected mainly by deforming the yoke on the flare (screen) side, where the six-pole component of the field has its greatest influence on the raster error.
The difference between local minima and maxima is preferably at least 0.2 mm. The number of maxima is preferably at least four, which has shown even greaater improvements of performance.
In accordance with a preferred embodiment, the inner and/or outer contour has a radius defined by the function
r0+xcex(acos(ixcex8)+bcos(ixcex8)), 
where a, bxcex5[0,1], i is an integer larger than 1, xcex is the amplitude, and r0 is the undeformed base radius. This implies that the deformation is not only periodic, but also harmonic, which has shown to be advantageous.
The inventive yoke ring may be mounted in a conventional deflection unit, which in turn may form part of a CRT.